A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is themost common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used. Fuel cells, unlike batteries, require a continuous source of fuel and oxygen/air to sustain the chemical reaction to generate an electromotive force (emf). Fuel cells continue to produce electricity for as long as these inputs are supplied.
A proton exchange membrane fuel cells (PEMFC) is a typical fuel cell design, where a proton-conducting polymer membrane (electrolyte) separates the anode and cathode sides, allowing charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit, producing the direct current emf. The startup time for a PEMFC is in the range of 1 second. Individual fuel cells produce relatively small electrical potentials, about 0.7 volts, so cells are “stacked”, or placed in series, to increase the voltage and meet an application's requirements. In addition to electricity, fuel cells produce water, heat and, depending on the fuel source, very small amounts of nitrogen dioxide and other emissions. The energy efficiency of a fuel cell is generally between 40-60%, or up to 85% efficient in cogeneration if waste heat is captured for use.
To deliver the desired amount of energy, the fuel cells are combined in series and parallel circuits to yield a higher voltage and/or current supply. Such a design is called a fuel cell stack. When the cell surface area is increased, stronger current can be drawn from each cell. In the stack, reactant gases must be distributed uniformly over all of the cells to maximize the power output.
In many practical applications, there are problems which contamination of the fuel cells, which reduces the operating life of the cells. Attempts have been made to lengthen the operating life of fuel cells by increasing the degree of purity of the hydrogen fuel, which increases the costs of manufacturing and distributing the hydrogen, and which will not eliminate certain problems that accumulates over time when the fuel cell is, as in automotive applications, repeatedly started and shut down, often for a prolonged period of time.
In automotive fuel cells systems some general operation principles are common and aimed at avoiding that the fuel cell stack is damaged by reverse current decay during shut-down. When automotive fuel cell systems have short shut downs the anode and cathode have hydrogen rich environment, as typically oxygen should be consumed from the cathode side during the shut-down process. This means that right after shut-down, there are mostly nitrogen and hydrogen present on the anode, while on the cathode there is mostly nitrogen and a small amount of oxygen. During a shut-down, an interchange of gases (oxygen, hydrogen and inert gases) through the membrane will occur. All the oxygen from the cathode or all the hydrogen from the anode is thus slowly consumed.
The problem is that during a longer shut-down, there will be slow diffusion of oxygen from both inlet and exhaust side to the cathode. This oxygen will eventually diffuse to the anode through the membrane, resulting in an oxygen atmosphere in both electrodes of the cells. When the system is started again and the main valve for hydrogen is opened, the anode system will typically contain a mix of oxygen, nitrogen, hydrogen and small amounts of other gases, until all other gases but hydrogen are purged out.
This mixing of gases causes a reverse current decay in the fuel cells, and was first reported by Reiser et. al. in Electrochemical and Solid-State Letters (2005), Volume 8, Number 6, pp. A273-A276. The electrolyte potential drops from 0 to −0.59 V when the anode is exposed to hydrogen and oxygen. This causes an opposite current flow to normal fuel cell mode at the oxygen-exposed region and raises the cathode interfacial potential difference to 1.44 V. This high voltage causes carbon corrosion which degrades the carbon carrier of the catalyst, which is usually a precious metal like platinum, or some other selected metals. In laboratory and in some stationary systems the problem can be solved by purging the anode system with nitrogen or some other inert gas before feeding in hydrogen. However, in automotive fuel cell systems there is no nitrogen source available.
Various solutions have been presented to overcome this problem. In WO 2010026819 is shown a fuel cell system that supplies predetermined amount of hydrogen held in a hydrogen supply flow channel to fuel pole side of fuel cell stack during system non-operation period. A similar solution is discussed in U.S. Pat. No. 8,232,014, disclosing a method for reducing the probability of air/hydrogen front in fuel cell stack, and involves injecting discrete amount of hydrogen into the anode side of fuel cell stack according to a predefined schedule. According to the method, hydrogen is periodically injected into a fuel cell stack after system shutdown to consume low levels of oxygen, as it diffuses back into the stack. These solutions are rather complicated and require separate control systems to monitor the conditions in the fuel cell and to inject hydrogen at an appropriate time and amount.
An alternative approach is presented in DE 102004042806, where is described a fuel cell system for a vehicle with catalytic converters in the inlet and outlet of the anode region to protect the anode from residual hydrogen by turning it into water. The hydrogen would otherwise diffuse into the anode while the fuel cell is turned-off and the region is filled with air/oxygen. This will however have no positive effect in the startup phase when hydrogen is forcedly entering the anode.
In WO 2011/039421 is presented a solution for general purification of fuel cell reactants by using slip stream filtering techniques. There a filter is added to the anode circulation to remove enriched impurities. Slip stream filters are an efficient and economical remedy to constantly clean streams of circulating liquids. Typically, a small stream of the liquid is directed into a bypass circulation containing a filter. Over time, most if not the entire volume of liquid will pass through the filter and become cleaned. A simple flow indicator beside the filter may allow the filter action to be gauged and announce the need to change or clean the filter.
When an inventive PEMFC system is started by first starting a gas circulation pump, then part of the anode gas (oxygen and nitrogen in the beginning) will flow though a slip-stream filter. When the filter is equipped with a catalytic burner, the oxygen in the anode gas can be consumed. By keeping the hydrogen flow smaller than what is needed to reduce the oxygen, there will not be residual hydrogen in the gas exiting from the filter before almost all oxygen is consumed, that would cause a problem addressed in the aforesaid publication DE 102004042806. A suitable amount of hydrogen to reduce the oxygen in the air (containing 21% O2), is 4% or less. The lower flammability limit of hydrogen in air at normal pressure (1 atm) is 4%, and should thus not be exceeded.
When all oxygen is consumed, there will be only hydrogen and inert gases left in the anode, and the main line for hydrogen can be opened without the problem of hydrogen/oxygen mixing and catalyst degradation. The burning process is inherently fast, taking only seconds to complete depending on the gas volumes involved and the circulation pump capacity in use, but this delay does not need to interfere with e.g. starting a car, as battery energy can be used during any fuel cell start-up delays.
According to the invention, an inventive system for eliminating reverse current decay in fuel cells comprises:                a fuel cell having an anode and a cathode;        a fuel feed system for supplying the anode of the fuel cell with fuel and forming an anode system;        a bypass line fitted in parallel and in flow connection with said anode system and capable of circulating fuel past the anode;        an oxygen reduction unit;        a pressure unit for circulating gas in at least part of said anode system and said bypass line.        
The bypass line is adapted to receive and circulate a flow of hydrogen during a fuel cell shutdown in order to mix the hydrogen with any oxygen present in the anode system, and to remove it from the anode system in said oxygen reduction unit by reducing the oxygen from the gas flow by catalytic conversion.
The present invention relies on the insight that it is possible to eliminate the oxygen from the anode side before letting the hydrogen in to the stack of the fuel cell. This can according to one embodiment be done by catalytic burning of oxygen. In a further embodiment of the invention, the oxygen burner or catalytic converter may be part of the bypass line. It may also be combined with a slipstream filter. Such a filter can be used for purifying hydrogen when the fuel cell is on, as shown in WO 2011/039421, and additionally to clean oxygen/nitrogen gases while the fuel cell is stopped. In this case no extra circulation systems in the hydrogen line are needed, but the oxygen present in the anode system can be burnt as part of the start-up procedure of the fuel cell.
Generally, the bypass line may be fitted between the main anode fuel feed line and the fuel return line in a loop, and it is branched from the main fuel line for the fuel cell and is provided with a valve for feeding an appropriate amount of hydrogen to the oxygen reduction unit in order to catalytically convert the oxygen present in the anode system into water.
The invention also encompasses a method for eliminating reverse current decay in fuel cells, comprising the steps of:                providing a fuel cell with a cathode and an anode and a fuel feed system for the anode forming an anode system;        providing a bypass line fitted in parallel and in flow connection with said anode system capable of circulating fuel past the anode;        feeding during a fuel cell shutdown condition a flow of hydrogen into said bypass line in order to mix the hydrogen with any oxygen present in the anode system:        circulating the gas flow in at least part of said anode system and said bypass line to an oxygen reduction unit;        reducing in said oxygen reduction unit any oxygen present in the anode system by catalytic conversion.        
The method may comprise an additional step of filtering the gas in the bypass line in order to remove any impurities from the gas flow. In one embodiment, the gas in the bypass line is circulated in a loop, which also includes the main anode fuel feed line and the fuel return line. An appropriate amount of hydrogen is fed and circulated through the bypass line to the oxygen reduction unit in order to catalytically convert the oxygen present in the anode system into water. Preferably, the required amount of hydrogen that is fed into the oxygen reduction unit is controlled by hydrogen and/or oxygen sensors that may reside at, before and/or after the oxygen reduction unit, or elsewhere in the circulation loop. In one embodiment, the oxygen consumption is sensed by measuring moisture, e.g. the water generation at the oxygen reduction unit. The more accurately the system can measure the amount of hydrogen and oxygen present in the system, the better the start-up of the fuel cell can be optimized and the start-up delay minimized.
The present invention is typically applied in a hydrogen fuel cell, preferably in a closed-end hydrogen fuel cell, but it can also be used in fuel cells using other fuels, in order to remove impurities essentially continuously. Most advantageously, the method and system are applied to clean the anode gas in a closed-end hydrogen fuel cell.
In the figures, the following reference numbers are used:    1 cathode    2 anode    3 hydrogen supply tank    4 main fuel valve    5 fuel return line    6 hydrogen feed line for oxygen reduction    7 valve for hydrogen feed to bypass line    8 bypass line    9 oxygen reduction unit    10 hydrogen/oxygen sensor    11 3-way valve    12 contaminant exit pipe    13 anode system purge valve    14 circulation pump    15 fuel (hydrogen) inlet to anode    20 control unit (computer)    21 sensor signal input line    22 actuator signal output line    23 actuator